1. Field of the Invention
The present invention relates to a circuit structure for controlling an element to be driven, such as an electroluminescence display element.
2. Description of the Related Art
Because electroluminescence displays that use a self-emitting electroluminescence (hereinafter, referred to as “EL”) element for each pixel as an emissive element are of a self light emitting type, and have advantages such as that the displays are thin and have low power consumption, a great deal of attention has come to be focused on such EL displays, and research is being conducted to use such EL displays as substitutes for displays such as liquid crystal displays (LCDs) or CRTs.
Among various types of EL displays, an active matrix type EL display, in which a switch element such as a thin film transistor (TFT) that individually controls an EL element is provided in each pixel to control an EL element for each pixel, is expected to be useful as a high resolution display.
FIG. 1 shows a circuit structure for one pixel included in an active matrix type EL display having a matrix with n rows and m columns of pixels. This EL display includes a substrate on which a plurality of gate lines GL are formed to extend in the direction of rows, and a plurality of data lines DL and a plurality of driving power supply lines VL are formed to extend in the direction of columns. Each pixel has an organic EL element 50, a switching TFT (first TFT) 10, an EL element driving TFT (second TFT) 21, and a storage capacitor Cs.
The first TFT 10 is connected to a gate line GL and a data line DL. When the gate electrode of the first TFT 10 receives a gate signal (selection signal), the first TFT 10 is turned on, and a data signal supplied to the data line DL is stored in the storage capacitor Cs that is connected between the first TFT 10 and the second TFT 21. A voltage corresponding to the data signal supplied through the first TFT 10 is supplied to the gate electrode of the second TFT 21, and the second TFT 21 is then used to supply a current corresponding to the voltage value from the power supply line VL to the organic EL element 50. The organic EL element 50 emits light when light emitting molecules excited by recombining holes injected from an anode and electrons injected from a cathode in a light emitting layer return from the excited state to the ground state. Because the luminance of an organic EL element 50 is approximately proportional to the current supplied to the organic EL element 50, when, as described above, the current to be fed to a organic EL element 50 is controlled in accordance with a data signal for each pixel, the organic EL element is caused to emit light at a luminance corresponding to the data signal so that a desired image is displayed on the whole display.
In order for an organic EL display to be able to achieve high display quality, it is necessary to cause the organic EL element 50 to emit light reliably at a luminance corresponding to the data signal. Therefore, in an active matrix type EL display, the second TFT 21 provided between the driving power supply line VL and the organic EL element 50 is required to provide a constant drain current even when a current is passed through the organic EL element 50 and varies the potential at the anode of this EL element 50.
Therefore, in many cases, as shown in FIG. 1, the second TFT 21 is formed using a p-ch TFT in which a source is connected to the driving power supply line VL, and a drain is connected to an anode side of the organic EL element 50 so that a current flowing between the source and the drain can be controlled based on a potential difference Vgs between the source and a gate to which a voltage corresponding to a data signal is applied.
However, when a p-ch TFT is used as the second TFT 21, because, as described above, a source is connected to the driving power supply line VL and a drain current, that is, a current to be supplied to the organic EL element 50 is controlled based on a potential difference between the source and a gate, so that there is a high possibility that the emission luminance of each element 50 will vary in response to variations in voltage on the driving power supply line VL.
For example, in cases such as where an image to be displayed in a certain frame period has a high brightness (as an example, where the color of an entire image is white), when a large amount of current is fed to a large number of organic EL elements 50 formed on a substrate at a time through respective driving power supply lines VL from a single driving power supply Pvdd, the potentials on the driving power supply lines VL may vary accordingly. Therefore, in such cases, variations in luminance are likely to occur.
With this being the situation, a pixel circuit wherein an n-ch TFT is used as shown in FIG. 2 to form the second TFT 20 for driving an element is proposed in a commonly assigned Japanese patent application laid open as Japanese Patent Laid-Open Publication No. 2003-173154. In this circuit, the second TFT 20 formed of an n-ch TFT is provided between a power supply line VL and an organic EL element 50, and, in order to hold a potential difference Vgs corresponding to a data signal between a gate and a source of the second TFT 20, a storage capacitor Cs is provided between the gate and the source of the second TFT 20. Further, a reset TFT 30 for resetting (discharging) the potential at the source of the second TFT 20 (the anode of the organic EL element 50) is connected between a low potential power supply Vss and each of the storage capacitor Cs and the source of the second TFT 20. A reset pulse is supplied to the gate of this TFT 30.
In such a structure, the potentials at the first and second electrodes of the storage capacitor Cs, or, in other words, the gate potential and the source potential of the second TFT 20, must be simultaneously set in response to a data signal. Thus, a selection signal having “H” level is output to a selection line GL, a data signal is output to a data line DL, and a reset pulse is output to a reset line RSL to turn on the TFT 30. As a result, the second TFT 20 is put into a state where the potential at the gate is set to a potential corresponding to the data signal, and the potential at the source is decreased to the potential of the power supply Vss. Further, when the first TFT 10 and the third TFT 30 are turned off, the storage capacitor Cs is held at a potential difference corresponding to the data signal. In response to the potential difference held in the storage capacitor Cs, it is possible to supply a current from the power supply line VL through the second TFT 20 to the organic EL element 50.
When a circuit structure as shown in FIG. 2 is used as described above, it is possible to use n-ch TFTs for all transistors formed in the circuit for one pixel. However, in order to set a potential at the source of the second TFT 20, because the third TFT 30 for discharge is also necessary, three transistors must be provided in each pixel. Further, the reset power supply Vss is necessary, and, in addition to the selection line GL, the data line DL, and the power supply line VL, the reset power supply line for supplying power to each pixel from the power supply Vss and the reset line RSL are also necessary. As a result, because the above-described circuit has a limit to how much the area of each pixel can be reduced, it is difficult to incorporate such a circuit into a compact high definition display or the like, such as an EVF (electronic view finder), in which each pixel has a small area.